FSFM261 Fairchild Semiconductor, FSFM261 Datasheet - Page 13

• Manufacturer Part Number: FSFM261
• Description: The FSFM260N/261N/300N is an integrated Pulse Width Modulator (PWM) and SenseFET specifically designed for high-performance offline Switch Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

PCB Layout Guide
Due to the combined scheme, FPS shows better noise
immunity than conventional PWM
controller and
MOSFET discrete solutions. Furthermore, internal drain
current sense eliminates noise generation caused by a
sensing resistor. There are some recommendations for
PCB layout to enhance noise immunity and suppress the
noise inevitable in power-handling components.
There are typically two grounds in the conventional
SMPS: power ground and signal ground. The power
ground is the ground for primary input voltage and
power, while the signal ground is ground for PWM
controller. In FPS, those two grounds share the same
pin, GND. Normally the separate grounds do not share
the same trace and meet only at one point, the GND pin.
More, wider patterns for both grounds are good for large
currents by decreasing resistance.
Capacitors at the V
and FB pins should be as close as
CC
possible to the corresponding pins to avoid noise from
the switching device. Sometimes Mylar® or ceramic
capacitors with electrolytic for V
is better for smooth
CC
operation. The ground of these capacitors needs to
Figure 23. Recommended PCB Layout
connect to the signal ground not the power ground.
Mylar® is a registered trademark of DuPont Teijin Films.
The cathode of the snubber diode should be close to the
drain pin to minimize stray inductance. The Y-capacitor
between primary and secondary should be directly
connected to the power ground of DC link to maximize
surge immunity.
Because the voltage range of feedback line is small, it is
affected by the noise of the drain pin. Those traces
should not draw across or close to the drain line.
In FSFM260/261/300, drain pins are the heat radiation
pins, so wider PCB pattern is recommended to decrease
the package temperature. Drain pins are also high
voltage switching pins; however, too wide PCB pattern
may deteriorate EMI immunity.
© 2009 Fairchild Semiconductor Corporation
www.fairchildsemi.com
FSFM260N / FSFM261N / FSFM300N • Rev. 1.0.1
13